1. Field of the Invention
This invention relates to fuel cell anodes, in particular electrolyte-particulate fuel cell anodes.
2. Description of Related Art
Fuel cells, as with batteries, generate useful energy by providing an oxidation reaction at a negative electrode/anode and a reduction reaction at a positive electrode/cathode. The electrical potential difference between the positive electrode and the negative electrode can be used to generate useful energy. Fuel cells can often involve one or more gaseous reactants. Gas diffusion electrodes, i.e., gas permeable electrodes, are suitable for use in electrochemical cells that have gaseous reactants, for example, for use in the cathode for the reduction of oxygen, bromine or hydrogen peroxide. The reduction of gaseous molecular oxygen can be an electrode reaction, for example, in metal-air/oxygen batteries, metal-air/oxygen fuel cells and hydrogen-oxygen fuel cells. Oxygen is generally conveniently supplied to these electrochemical cells in the form of air. The oxidation reaction at the anode gives rise to the electrons that flow to the cathode when the circuit connecting the anode and the cathode is closed.
The electrons flowing through the closed circuit enable the foregoing oxygen reduction reaction at the cathode and simultaneously can enable the performance of useful work due to an over-voltage between the cathode and anode. For example, in one embodiment of a fuel cell employing metal, such as zinc, iron, lithium and/or aluminum, as a fuel and potassium hydroxide as an electrolyte, the oxidation of the metal to form an oxide or a hydroxide releases electrons. In some systems, a plurality of cells is coupled in series, which may or may not be within a single fuel cell unit, to provide a desired voltage. For commercially viable fuel cells, it is desirable to have electrodes that can function within desirable parameters for extended period of time on the order of 1000 hours or even more.
The present invention relates to an improved electrolyte-particulate fuel cell. The fuel cell comprises a cathode and an anode through which electrolyte flows, for example, from top to bottom. The fuel cell can include a mesh or screen where the screen is located near the end of the anode along the flow, such as the bottom of the anode for flow from the top to the bottom, and where the screen has a surface area greater than the anode""s bottom cross-sectional area. In one embodiment, the screen has a surface area that is at least 40 percent greater than the anode""s bottom cross-sectional area. The screen may an expanded metal or polymer mesh, a woven metal or polymer mesh, or a perforated metal or polymer sheet. In some embodiments, the screen may comprise a series of parallel ribs.
In a fuel cell where an electrolyte-particulate based fuel flows along its anode, larger particles gradually dissolve within the anode and participate in energy generation. The particles form a static bed that is gradually consumed and replenished while the electrolyte generally is in continuous flow. The dissolving, smaller particles of the electrolyte-particulate fuel may congregate at the anode bottom and thereby reduce the fuel flow rate and current density for the fuel cell. The anodes described herein provide an electrolyte-particulate fuel cell anode that improves the electrolyte flow rate with reduced particulate blockage at the anode bottom.
In one embodiment the particulate has an initial pre-consumption approximate size and becomes smaller as it flows along the anode, for example, from the cell top to bottom, under electrical load. In this embodiment, the screen has a plurality of openings where at least one of the plurality of openings has an area size that enables some unconsumed and some partially consumed particulate to pass. In one exemplary embodiment, the particulate is zinc particles and the electrolyte is potassium hydroxide. Further, the pre-consumption zinc particles may have a diameter of about 0.5 mm. In one embodiment the cell width is about 2.0 mm. In another embodiment at least one of the plurality of screen openings has a width of about 0.6 mm and a height of about 1.0 mm. In another embodiment at least one the plurality of screen openings is circular. In a further embodiment at least one of the plurality of screen openings has a width of about 0.6 mm and a height of at least 2.5 mm, in other embodiments at least one of the plurality of screen openings has dimensions of 2 mm by 2 mm and in further embodiments, at least one of the plurality of screen openings has dimensions of 1.2 mm by 2 mm.